I've been told by a friend who finished UN a couple of weeks ago that you can only get the two new zombie hunter costumes after you beat the game. Now, I beat it an hour ago, and I went to save right after Zombie Marston crawls out of his grave. However, when choosing the "Change Outifts" option, all the things are blacked out. Can I still get these outfits somehow, or was my friend just bullshitting me, and will I have to reload a save to before the end of the game to get them?

You can get the costumes before you beat the story. The full requirements are attainable as early as you gain access to Mexico infact. 
 
As for the costumes blacking out, that's just a glitch that happens when you first check out the costumes. Check again and you'll see all three of Zombie Marstons costumes again.